Escherichia coli Justin Bosch
By: Justin Bosch
After many years of extensive research we now know that E. coli is a rod-shaped bacterium that is capable of respiring both aerobically and anaerobically depending on the presence of oxygen. This ability is more commonly known as a facultative anaerobe. We also know that it stains Gram-negative due to the bacterium's cell wall and outer membrane obtaining the color from safranin, a red counterstain. The cell wall also protects the bacteria from antibiotics like penicillin .
E. coli is not a picky bacterium; they can live on a plethora of substrates and laboratory media due to their ability to deal with anaerobic and aerobic conditions. They also can grow and reproduce at a wide range of temperatures; however, Escherichia coli's optimal temperature for reproduction is around thirty-seven degrees celsius.
Additionally, E. coli is an incredibly diverse species, both genetically and phenotypically. As a matter of fact, amidst all strains of E. coli, only around twenty percent of their genome are mutual or shared. Although most of these unique differences may only be distinguishable at the molecular level, they may have effects on a larger scale; such as, alter the organism's physical makeup. The vast amount of differences in this species allows for it to adapt to its distinct environment possibly; for example, many strains of the bacteria have grown to be host-specific. Some strains also have adapted to be resistant to antimicrobial agents.
Even though there is a considerable amount of total E. coli strains in the world, most strains are known to reside in the intestine of warm-blooded organisms like humans. Most strains are harmless; matter of fact, they are known to aid the health of their host, especially during digestion as a part of a symbiotic relationship. For instance, Escherichia col is known to facilitate and help the health of humans, particularly the health of its immune and digestive systems. E. coli produces certain probiotics that improve our immune responses by providing and stimulating the production of antibodies in the gut to counter a plethora of conditions, like diseases and infections. Therefore, with E. coli protecting the small intestines, our immune system does not need to do so, allowing for it to combat other infections and diseases in other parts of the body.
As we know, this species counteracts many conditions, such as infections in the gut. However, this bacterium does much more to aid intestinal problems. For example, E. coli promotes digestion by creating digestive enzymes that help in breaking down the food in our intestines, allowing for the absorption of the critical nutrients of our food to be facilitated. Therefore, promoting a usual bowel function provides for the discomfort of constipation and infectious diarrhea to be reduced. As well as ridding the gut of the various toxins and waste dwelling in the body, thus, decreasing bloating. All in all, E. col is immensely important to the health of the mammalian digestive system.
Although Escherichia coli can be an essential factor in human health, it can also be harmful. E. coli is also known to cause severe food borne illnesses, especially from eating undercooked meat, as well as fecal contamination of consumables like water. However, E. coli can cause a wide variety of diseases and infections. For instance, "one of the most frequent causes of many common bacterial infections, including cholecystitis, bacteremia, cholangitis, urinary tract infection (UTI), and traveler's diarrhea, and other clinical infections such as neonatal meningitis and pneumonia." . In the United States, up to half of the female population experiences a UTI caused by E. coli once in their lifetime. When traveling to developed areas, the bacteria is the cause of diarrhea in up to nearly fifteen percent of the visitors.
The most common bacterial infection is urinary tract infections; more often than not, these infections are caused by E. coli, particularly the uropathogenic strains. Uropathogenic organisms are pathogens that reside or are related to the urinary tract. These specific strains of E. coli cause a wide range of UTIs like cystitis by binding to P blood antigens and intervene between the attachments of urothelial cells and E. coli. Although, most E. coli causing urinary tract infections are not complicated. However, some may lead to some complications and may lead to more severe outcomes; for example, disseminated intravascular coagulation and death.
Another disease caused by E. coli is acute bacterial meningitis, especially prevalent in newborn children; This disease is caused by a specific strain of E. coli known as K1. Neonatal sepsis is an infection of the bloodstream caused by bacteria. It is known to have a relatively high mortality rate at around eight percent.
E. coli intra-abdominal infections may be caused by many things like abscesses, cholecystitis, and none more likely than a perforated internal organ like an appendix. Escherichia coli liver abscesses are common, particularly in those with previous health issues like diabetes. Gastrointestinal punctures or anastomotic procedures may cause spillage of the contents from within the large intestine, therefore, leading to abscesses in the abdominal region. Intra-abdominal abscesses are known to be polymicrobial, clusters of viruses, bacteria, fungi, etc., perfect for E. coli with it being able to survive in vast and broad environments. E. coli also thrives and causes intra-abdominal infections because the obstruction of the biliary system promotes bacterial growth. The biliary system may be fully or partially blocked from stones and waste build-up; matter of fact, partial obstructions cause more frequent infections.
Escherichia coli, also known as E. coli, was first observed by Theodor Escherich in 1885. The pediatrician detected the microbe in the feces of his healthy patients; he then named it Bacterium coli commune because it was found in the colon.
Because of the vast amount of medical benefits, E. coli has been a centerpiece of scientific studies since it was first discovered to the present. In 1946 Escherichia coli was used first to detect bacterial conjugation , Edward Tatum and Joshua Lederberg. "Bacterial conjugation is a sexual mode of genetic transfer in the sense that chromosomal material from two sexually distinct types of cells are brought together in a defined and programmed process." . To this day, E. coli remains the primary model for studying bacterial conjugation soon after Seymour Benzer completed experiments to study the gene structure of T4 and E. coli because no one knew whether the genes were using branching patterns or linear structures.
Many studies have been performed using E. coli since then. For example, A more recent study was done in 2009 workers were separated into two groups to test medical benefits. The study found that workers that took the E. coli instead of the placebo had a smaller amount of respiratory and digestive issues compared to those that did not.
A plethora of experiments have been done about E. coli, none more prominent in microbial science than that of Richard Lenski . This study began in 1988 and continues to this day in order to observe the evolution of the bacterium and its different populations. This study has produced various findings and data, one being that a particular strain and population of Escherichia coli can digest citrate, a derivative of citric acid.
Section 6 (evolution) As mentioned before, Richard Lenski and his colleagues studied the E. coli long-term evolution experiment from 1988 to the present . This study tracked twelve indistinguishable populations of E. coli, until recently Lenski put the research on pause due to the COVID-19 outbreak. The communities were proliferating; by 2010, a total of sixty-six thousand generations were studied and observed. Over the years, the evolving populations have drastically changed, both phenotypically and genotypically. Some of these changes developed in all twelve populations, such as increased cell size, faster growth rates, and improved fitness that eventually diminished over time. On the other hand, some changes only took place in a few populations, like the inability to repair DNA. A single population also evolved to be able to grow on citrate aerobically.
At the start of this ongoing experiment, Lenski used the E. coli strain Bc251. This strain was previously studied and much understood by many scientists before him, like Seymour Lederberg. This strain was then known to have many distinct and defining traits; for example, the inability to grow on arabinose. Lenski found six Ara− colonies in strain Bc251; these populations are known as Ara-1 through Ara-6. He then found six Ara+ colonies in a variant of the Bc251 strain, labeling these Ara+1 through A+6. These markers allow for the populations to be differentiated on their plates in the lab.
The results of this experiment show that the populations’ fitness has changed compared to the original strains. All twelve populations of E. coli saw a dramatic increase in their relative competence. However, over time their fitness has been slowing down. At around the twenty thousand generation mark, the populations grew at an extraordinary rate, specifically seventy percent faster than the first generation. This increase in fitness seems to fit a power-law model instead of a hyperbolic model that was used in previous years. “A power law is a relationship in which a relative change in one quantity gives rise to a relative proportional change in the other quantity, independent of the initial size of those quantities. An example is the area of a square region in terms of the length of its side. If we double the length, we multiply the area by a factor of four.” . Therefore, according to the data, the rapid growth and fitness of the populations will continue to increase unless mutations occur to lower fitness levels.
Half of the populations have acquired an inability to repair DNA, causing the mutations to be more prevalent. Within the first twenty thousand generations, Lenski believed that each community underwent less than one hundred total point mutations, including around ten to twenty beneficial mutations, therefore reaching fixation in all twelve populations. There were studies done that observed the genome from multiple points in time of a specific population. This study found that the mutations were linear and cyclical like a clock, although most of the variations were found to be beneficial instead of neutral.
Like the increase in fitness, all populations in the study increased in cell size according to the findings. This increase in cell size coincided with the decrease in population density. Most cells also took more of a round shape instead of their standard rod shape. This change in size and shape had somewhat to do with the difference in the gene expression of a penicillin-binding protein. Therefore, causing the mutant strains to be able to outcompete past generations. Not only did this mutation increase fitness in the long-term evolution experiment’s conditions, but it also diminished their survival rate in stationary phase cultures due to the increase in sensitivity of osmotic stress.
Throughout the experiment, the populations were able to adapt and specialize in their environment and the resources they were growing on, which happened to be glucose. For example, in the year two thousand, Cooper and Lenski’s results informed us that all twelve populations of E. coli had a smaller range of substances on which it could survive and grow successfully due to a lack of unused metabolic functions after a plethora of generations. Therefore, mutations in all populations disposed of the ability to survive on other substances, that is not the sole substance they have been growing on. Their findings suggest that an abundance of neutral adaptations and mutations in unused genome sequences instead of antagonistic pleiotropy. Antagonistic pleiotropy is the loss of unused adaptations. Therefore, these later studies show that adapting to a single environment or substance does not always end up with specialization.
Authored for BIOL 238 Microbiology, taught by Joan Slonczewski, 2018, Kenyon College.
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